salt solution. By raising each test separately above the screen so that
the upper part becomes directly illuminated it is easy to discern the cloud
produced by the new addition of solution.
Reagents. I. Standard salt solution, 100 c.c. of which precipitates almost
completely i gram of pure silver. It is prepared by dissolving 5-4200
grams of pure sodium chloride to i litre with distilled water or 5-570 grams
of sea-salt, dried between filter papers, to i litre with ordinary water 1 and,
in the latter case, filtering the solution.
2. Weak standard salt solution, one-tenth as strong as the preceding
solution, from which it may be prepared by dilution J or 0-5420 gram of
pure sodium chloride may be dissolved to i litre. This solution, i c.c. of
which corresponds with o-ooi gram Ag, is stored in a bottle fitted with a
rubber stopper traversed by a pipette graduated from i to 5 c.c.
3. Pure nitric acid, D 1-2, free from chlorine.
Standardisation of the salt solution, i gram of pure silver (fine silver,
see preceding method) is weighed with the greatest accuracy 2 and heated
in a water-bath in one of the test bottles with 8-10 c.c. of nitric acid (D 1-2)
until the metal is dissolved and the red vapours have disappeared. After
cooling, the neck"of the_bottle is washed with a few drops of water and 100
c.c. of the standard salt solution introduced by means of one of the pipettes
described, care being taken that only the liquid falling in a continuous
stream enters and not the subsequent drops. The bottle is then stoppered
and shaken for about 10 minutes in the shaking apparatus, the precipitate
clotting and the liquid becoming quite clear. With a rapid shake the
particles of precipitate are removed from the upper part of the bottle, the
latter being then placed on the bench, the stopper removed and i c.c. of
the weaker standard salt allowed to flow gently down the side of the bottle.
After 4-5 minutes the bottle is raised so that the upper portion of the liquid
becomes illuminated, fresh precipitation of the silver in the form of a cloud
at the surface of the liquid being usually observed.
The solution is exact when this cloud is barely perceptible and when
it disappears on gently shaking the liquid ; if there is too much cloud, the
standard solution must be corrected by addition of salt, whilst, if no cloud
is produced, the solution must be diluted. Only the first case will be con-
sidered, as the second may be reduced by suitable dilution to the first.
When the addition of i c.c. of the weaker standard salt produces too intense
a cloud, the bottle is shaken in the apparatus for 10 minutes and, after
clearing, treated with a further i c.c. of the weaker salt. This process is
continued until such an addition causes either no further precipitation or
only a scarcely perceptible cloud. In calculating the correction, the last
c.c. is either neglected or taken as only 0-5 c.c. in the first case, but must
be taken into account in the second case. If, for instance, the complete
1 In assaying laboratories sea-salt is usually employed and 10 litres of solution
prepared at a time.
2 To facilitate weighing, strips of the rolled metal weighing more than i gram are
placed on the balance pan, the excess being then removed first with metal shears and
then by rubbing one of the strips (the largest) held in flat-ended tongs against a very
fine file until perfect equilibrium of the balance is attained. The filed piece should
be dusted with a brush to remove any adherent filings.o high,